1. Field of the Invention
The present invention relates to a method for determining at least the polarity of the direction of remanent magnetization of an earth formation penetrated by a bore hole so as to indicate the geologic age of a segment of the formation and to document stratigraphic boundaries occurring, say, during generation and migration of petroleum over a given span of geologic time such as when accumulative traps were generated.
Geologic dating of the relevant adjacent sections of strata within a bore hole by conventional means is both time-consuming and costly. Use of in-hole dating equipment such as magnetometers and the like has not been successful in age-dating due to inaccuracy of the generated results. Use of cores of sediments, i.e., long cylinders of successive layers of sediment, is likewise costly and requires extensive well time to accomplish.
2. Background of the Prior Art
It is well known in carrying out geologic dating that the magnetization of a section of earth formation is associated with magnetic fields that are intrinsic to that body and act at a distance from it. Since the direction and strength of the field can be identified, there can be gained an indication of both origin of the body and, hence, its geologic age assuming the field of interest can be accurately determined.
That the magnetization of a given sample volume of an earth formation is a source of potential magnetic field in accordance with the following equation is well known: EQU M = M.sub.R + KH.sub.o
Where M equals the magnetization or magnetic dipole moment per unit volume of a sample;
M.sub.R equals the natural remanent magnetization of the sample; and
KH.sub.o equals the induced magnetization of the sample.
In contrast with the above known functional relationship, the measurement of the contribution of the natural remanent magnetization can be difficult to obtain since measurements are carried out in the presence of the earth's field. Not only is the intensity of the anomaly due to the remanent magnetization small in comparison with the induced magnetization, but also its direction is dependent upon the paleomagnetic origin of the sample as well as subsequent movement of the sample remote from the origin as a function of geologic time, ("continental drift").